32 EVOLUTION AND DISEASE. 
That increased size is not due always to increased — 
function but may arise from irritation is admirably 
illustrated in Solanum jasminoides described by Darwin. 
“ The flexible petiole of a half or a quarter grown leaf 
which has clasped an object for three or four days 
increases much in thickness, and after several weeks 
becomes so wonderfully hard and rigid that it can hardly 
be removed from its support. On comparing a thin 
transverse slice of such a petiole with one from an 
older leaf growing close beneath, which has not clasped 
anything, its diameter was found to be fully doubled 
and its structure greatly changed ” (“Climbing Plants”). 
In this example the extra thickness could not be due 
to increased function, but to irritation ; the petiole had 
less work to perform as the leaf was largely supported by 
the object which its petiole had clasped. } 
The effects of increased use may be observed in the 
organs of special sense. When an individual loses an 
eye in early life the remaining healthy eye acquires a 
creater range of movement and quickness which com- 
pensates in no small degree for the loss of its companion. 
In persons blind from early life the power of hearing 
becomes wonderfully quickened, and their tactile sensi- 
bility is so heightened that they make themselves 
acquainted with external surroundings in a marvellous 
manner. Similar instances are furnished by the mole ;_ 
its sense of hearing is proverbial. Says Caliban to 
Stephano and Trinculo: “ Pray you tread softly, that 
the blind mole may not hear a footfall.” The blind 
fish of the mammoth cave are said to be abnormally 
sensitive to sounds as well as to undulations produced | 
